Hello i have a registration and login form on my website.

In the top right corner or whereever on my website i want the following text to be present:

Welcome, Guest!
The 'Guest' should be replaced with the logged in user. This text will be on every page. How do i change the text so it is customized for the logged in user? I was thinking since PHP is server side if i were to make a change for one page it would make the same change for every user viewing the page.

How would i go about this?